Physical Properties of Water
Mass density (

) = mass/unit volume (kg/m
3
)
Specific weight (

) = force/unit volume (kN/m 3 )
Specific gravity (S) =
 wo
or
 wo
(relative to water at 3.98

C)
Types of Solutions
Dissolved: substance (solute) dissolved and homogeneously dispersed in liquid
(solvent)
Suspended: solids large enough to settle by gravity or be removed by filtration
Colloidal: between dissolved and suspended; can be filtered out but will not settle by gravity
Composition of Solutions
Two systems of measurement:
1) mass/mass (weight/weight): e.g., mg/kg, ppm
2) mass/volume (weight/volume): e.g., mg/L, ppm*
*mg/L and ppm often used interchangeably – true if S=1, otherwise must convert using X ppm (mg/kg)= Y mg/L (1/S) mass/mass (weight/weight) examples: a) percent by weight b) ppm (parts per million) c) molality, m = # gram moles solute per 1000 g solvent d) mole fraction, x
I
= (number moles solute, n)/(total number of moles, n
T
) mass/volume (weight/volume) examples: a) mg/L b) molarity, M = # gram moles solute per L solution (g mol / L) c) normality, N = # gram equivalent weights of solute per L solution
(eq/L) d) meq/L: expression of normality for very dilute solutions, 1 eq/L = 1000 meq/L
Normality : a given substance can have more than one gram equivalent weight, depending on the reaction it undergoes. Therefore, when expressing the

composition of a solution using normality, one must specify the reaction or type of reaction it is used in.
Equivalent weight (g/eq) = # g per mole (g) / n (equivalents per mole)
Where n= # protons donated in acid-base reaction or total change in oxidation number in oxidation-reduction reaction
Also, N=Mn
Concentration in terms of a Common Constituent
*used in water chemistry but typically not in general chemistry* example using Nitrogen compounds
Forms of Nitrogen: Ammonia: NH
4
+ , NH
3
Organic:
Nitrite:
Nitrate: various
NO
2
-
NO
2
-
Wastewater sample: Ammonia 30 mg/L NH
3
Nitrite
Nitrate
0.1 mg/L NO
2
-
1.5 mg/L NO
2
-
Organic 5.0 mg/L N
Convert to N: Ammonia 30 mg/L NH
3
(14/17) = 24.7 mg/L as N
Nitrite 0.1 mg/L NO
2
-
(14/46) = 0.03 mg/L as N
Nitrate 1.5 mg/L NO
2
-
(14/62) = 0.35 mg/L as N
Organic
TOTAL
5.0 mg/L N (14/14) = 5 mg/L as N
30.07 mg/L as N
Chemical Reactions : four types most common in Environmental Engineering
- precipitation
- acid-base
- ion-association
- oxidation-reduction (redox)
Precipitation Reactions : dissolved ions react to form solid compound
Solubility equation: A a
B b
(s)
⇋
aA b+
+ bB a-
A b+
(cations) attracted to (-) ends of water dipoles
B a-
(anions) attracted to (+) ends of water dipoles
The product of the activity of the ions is always constant for a given compound at a given temperature at equilibrium
K sp
= solubility product = [A] a
[B] b
PK sp
= - log(K sp
)

Acid-Base Reactions
- carbonate system
- concentration of metal ions
- water softening
- relation to precipitation and redox reactions
Lowry-Bronsted definition: acids are substances that tend to lose (donate) protons (H
+
), bases tend to gain (accept) protons
*for an acid-base reaction, must have both donator and acceptor*
Ionization of water: water is weakly and reversibly ionized
Strong acids: tendency to completely donate protons to water
Weak acids: do not completely dissociate
Buffering: a buffer solution resists large pH changes when an acid or base is added
Carbonate system: most important
Carbon dioxide, CO
2
(aqueous or gaseous)
Carbonic acid, H
2
CO
3
Bicarbonate ion, HCO
3
-
Carbonate ion, CO
3
2-
Carbonate-based solids, calcium and magnesium
Alkalinity: measure of water’s ability to neutralize acids
In most waters, only significant contributors to alkalinity are carbonate species and free H
+
or OH
-
Serves as a measure of buffering capacity
Note difference between alkaline water and water with high alkalinity
Alkalinity as mg/L CaCO
3
(by convention, do not use molarity units) mg/L as CaCO
3
= mg/L as species (EW CaCO
3
)/(EW species)
Ion Association Reactions : ions may exist in water that are “complexed”
(electrochemically tied) with other ions, causing them to behave differently
Oxidation-Reduction Reactions : redox reactions involve valence (charge) changes and the transfer of electrons
Oxidation: increase valence (lose electrons)
Reduction: decrease valence (gain electrons)

Reaction Kinetics : reactions function of temperature, pressure, concentrations of reactants (law of mass action)
*previously discussed derivation, order, determination of reaction rate constant (k)
Gas Transfer: important time-dependant reaction
Example: replenishment of oxygen used by bacteria in depleting organic waste is accomplished by oxygen transfer from air to water (natural or artificial)
Solubility: Henry’s Law
X

P
H
X = equilibrium mole fraction of dissolved gas
= (moles gas, n) / (total moles, n
T
)
P = partial pressure of gas in equilibrium with liquid, atm
H = Henry’s law constant, atm -1
Dalton’s Law: each gas in a mixture exerts a partial pressure in proportion to its percentage by volume in the mixture:
P*Vol. = (p
1
+p
2
+…p n
)*Vol. where P=
 p i so x i
 p
H i i also x i
 n n
T i
 n
1
 n
2
 n i
  n n
 n w
Transfer Rate dC
 k a
( C s

C ) dt